1. Field of the Invention
The present invention relates to an inkjet printer and a method for printing.
2. Discussion of the Background
Inkjet printers that perform printing by discharging an ink on a printing medium from a printer head while reciprocating the printer head over the printing medium are known in the art. Some of these inkjet printers perform printing with an ultraviolet curing ink (hereinafter, “UV ink”) that dries when exposed to an ultraviolet radiation. Because the UV ink is better in weather resistance and water resistance, a printed material printed with the UV ink can be suitably used as publicity handouts for outdoor advertising, and such printed material is advantageous in having a wide range of applications than a printed material printed with a water-base ink. An inkjet printer that performs printing with the UV ink typically includes an ultraviolet radiation device for drying the UV ink adhering to the printing medium. Recently, inkjet printers have been developed that employ an ultraviolet light emitting diode (hereinafter, “UVLED”) as a light source for generating an ultraviolet radiation in the ultraviolet radiation device.
A conventional printing unit 500 mounted on an inkjet printer is shown in FIG. 9A. For explanation purpose, directions shown with arrows in FIG. 9A will be called a front-back direction and a left-right direction. The printing unit 500 mainly includes a printer head 510 that discharges a UV ink, a right ultraviolet radiation device 520R, a left ultraviolet radiation device 520L, and a carriage 530 for mounting these components. The right ultraviolet radiation device 520R and the left ultraviolet radiation device 520L are arranged on a left side and a right side of the printer head 510, respectively. Each of the right ultraviolet radiation device 520R and the left ultraviolet radiation device 520L internally includes a UVLED, and emits an ultraviolet radiation in a lower direction.
When performing printing on a printing line 508 on a printing medium 501, the UV ink is adhered to the printing line 508 in a desired pattern by discharging the UV ink from the printer head 510 while causing the carriage 530 to perform a reciprocating movement (hereinafter, “pass”) above the printing line 508 in the left-right direction. Because of the ultraviolet output from the right ultraviolet radiation device 520R and the left ultraviolet radiation device 520L, the UV ink adhering to the printing line 508 is cured when the ultraviolet radiation falls on the printing line 508, and results into printing.
FIG. 9B shows a cross section of the UV ink in a situation where an intensity of the ultraviolet radiation output from the right ultraviolet radiation device 520R and the left ultraviolet radiation device 520L is relatively high. FIG. 9B shows a situation where a UV ink 512 is discharged and adhered in a current pass onto a completely cured UV ink 511 that was discharged, adhered, and completely cured with the ultraviolet radiation in a previous pass. Because the UV ink 512 has bad affinity (difficult to become flat) for the completely cured UV ink 511, the UV ink 512 swells as a granule due to its surface tension, almost does not spread, and is cured in this situation with the ultraviolet radiation.
FIG. 9C shows a cross section of the UV ink in a situation where the intensity of the ultraviolet radiation output from the right ultraviolet radiation device 520R and the left ultraviolet radiation device 520L is relatively low. FIG. 9C shows a situation where a UV ink 514 is adhered in a current pass onto a non-cured UV ink 513 that was adhered but not cured in a previous pass. Because the non-cured UV ink 513 has a good affinity for the UV ink 514, the two inks mix readily and they are cured in this situation with the ultraviolet radiation. Such mixing of the inks can lead to degradation in a printing quality. To take care of this issue, Japanese Patent Application Laid-open No. 2004-276584, for example, discloses a technique in which only a surface of an ink adhering onto a recording medium 2 is first cured with an ultraviolet radiation output from first radiation devices 17, 18, 19, and 20, and then, the ink is completely cured with an ultraviolet radiation output from a second radiation device 21. Moreover, FIG. 1 of Japanese Patent Application Laid-open No. 2008-284708 discloses an inkjet recording apparatus 1 including two light sources 19 and 20 having mutually different illuminances.